Dynamic braking system for alternating current induction type motors



Oct. 29, 1968 D. E. GRAHAM 3,408,550 DYNAMIC BRAKING SYSTEM FORALTERNATING CURRENT INDUCTION TYPE MOTORS Filed Jan. 11, 1966 Direct 3 2Variable current Voltage potential motor speed Sol/r06 control circuitcurrent potential source 70 reference phase of 4/6 supp/y potentialINVENTOR. Dona/o 5. Graham l-lis Attorney United States Patent ABSTRACTOF THE DISCLOSURE A system for dynamically braking an alternatingcurrent induction type motor, the speed of which is controlled by avariable voltage motor speed control system which includes a motor speedcontrol potentiometer. Respective potentiometers are connected in shuntacross a source of direct current reference potential and a directcurrent tachometer generator driven by the motor, the movable contact ofthe potentiometer connected across the source of reference potentialbeing mechanically interlocked with the movable contact of the motorspeed control potentiometer. Upon an adjustment of the motor speedcontrol potentiometer movable contact to change motor speed, adifference potential signal appears across the movable contacts of thepotentiometers connected across the source of reference potential andthe tachometer generator which activates responsive circuitry tocomplete an energizing circuit for a separate brake winding wound uponthe stator of the motor across a separate source of direct current brakepotential.

This invention relates to braking systems for electrical motors and,more specifically, to a dynamic braking system for alternating currentinduction type motors.

With variable voltage speed control systems for alternating currentinduction type motors, the speed of the motor is controlled by adjustinga motor speed control device which controls the time rent supplypotential. To increase motor speed, the time during which full linepotential is applied to the motor is increased by increasing theconduction angle and to reduce motor speed, the time during which fullline potential is applied to the motor is reduced by reducing theconduction angle. Therefore, when speed control systems of this type areadjusted to reduce the speed of the motor, the speed of the rotor of themotor does not reduce in proportion to the adjustment of the motor speedcontrol device because of rotor inertia.

For precise and accurate speed adjustment of alternating currentinduction motors which are controlled by variable voltage speed controlsystems, it is necessary that the rotor speed reduces with theadjustment of the motor speed control device so that the rotor is alwaysrotating at substantially the speed corresponding to the setting of thesystem motor speed control device. Therefore, a system which dynamicallybrakes the rotor of a variable voltage controlled induction motor, inresponse to an adjustment of the motor speed control device of thevariable voltage motor speed control system to reduce the speed of themotor, is highly desirable.

It is, therefore, an object of this invention to provide an improvedbraking system for alternating current induction type motors.

It is another object of this invention to provide an improved dynamicbraking system for alternating current induction type motors.

It is a further object of this invention to provide an improved dynamicbraking system for alternating current induction type motors term toreduce the speed of the motor.

In accordance with this invention, a dynamic braking system foralternating current induction type motors is provided wherein a brakewinding, wound upon the stator of the motor, is energized by a source ofdirect current brake potential in response to a difference signal whichis and 12, respectively.

In a practical application, the novel dynamic braking As the point ofreference or ground potential is the same point electrically throughoutthe system, it has been illustrated by the accepted symbol andreferenced by the numeral 5 throughout the figure.

The novel dynamic braking system of this invention a difference signalwhich is promotor, to energize a stator of the motor by potential.

The source of direct current brake potential, shown in schematic form inthe figure and referenced generally by numeral 14, may include a fullWave diode bridge type rectifier having two alternating current inputterminals 15 and 16 and two direct current output terminals 17 and 18.Brake winding 26 is connected across direct current output terminals 17and 18 and alternating current input terminals 15 and 16 are connectedacross the secondary winding 20 of transformer 21, the primary The motorspeed control device of the variable voltage motor speed control systemwhich may be adjusted to control the speed of motor 12 may be a motorspeed control potentiometer shown in the figure as a conventionalpotentiometer 24 having a movable contact 25.

nally of variable voltage motor speed control circuit 10 and motor 12,respectively, to facilitate the description of the novel dynamic brakingsystem of this invention.

The difference signal which is produced in response to an adjustment ofthe variable voltage motor speed control system to reduce the speed of acontrolled motor may be the difference in magnitude between a referencepotential, the magnitude of which is changed in response to anadjustment of the control system motor speed control device to reducethe speed of the motor, and a control potential, the magnitude of whichvaries with motor speed.

To provide the reference potential, a conventional potentiometer 28 maybe connected in shunt across the positive and negative polarityterminals of a source of direct current reference potential 32 throughend terminals 29 and 30, respectively. As this source of referencepotential may be any source of direct current potential well known inthe art and forms no part of this invention, it has been illustrated inblock form in the figure. Movable contact of motor speed controlpotentiometer 24 may be mechanically interlocked with movable contact 31of potentiometer 28 in such a manner that, upon an adjustment of movablecontact 25 to reduce the speed of motor 12, movable contact 31 isadjusted in a direction away from end contact 29 toward end contact 30,in a manner well known in the art.

To provide a control potential which varies with motor speed, anelectric tachometer which may be driven by the motor shaft and whichproduces a direct current output potential, the magnitude of which isproportional to motor speed, may be employed. Commercially availabletachometers of this type are small direct current generators which aredesigned to produce an output direct current potential which increasesand decreases linearly with increases and decreases of motor speed.These devices are adapted to be mounted upon the end bell of the motorhousing in such a manner that the armature thereof is rotated by themotor shaft. As electrical tachometers of this type are well known inthe art and form no part of this invention, it has been schematicallyillustrated as a circle in the figure and referenced by the numeral 33.A conventional potentiometer 34 is connected in shunt across the outputterminals of tachometer 33 through end terminals 35 and 36.

Movable contacts 31 and 37 of potentiometers 28 and 34, respectively,are adjusted relative to each other to produce equal potentials betweenmovable contact 31 and fixed contact 30 of potentiometer 28 and movablecontact 37 and fixed contact 36 of potentiometer 34 at a selected motorspeed which may be synchronous speed or any other speed less thansynchronous speed.

Upon the adjustment of movable contact 25 of motor speed controlpotentiometer 24 in a direction to reduce the speed of motor 12,mechanically interlocked movable contact 31 of potentiometer 28 is movedin a direction toward fixed contact 30 and a difference signal appearsacross movable contacts 31 and 37 which is of a positive polarity uponmovable contact 37 relative to the potential upon movable contact 31because movable contact 31 has been adjusted in a direction toward thenegative terminal of source of reference potential 32.

To energize brake winding 26 by the source of direct current brakepotential 14, circuitry responsive to the difference signal produced bythe arrangement hereinabove described for establishing an energizingcircuit for brake winding 26 across source of direct current brakepotential 14 is provided.

Connected in series with brake winding 26 across the direct currentoutput terminals 17 and 18 of brake potential source 14 is acontrollable switching device, having two current carrying electrodesand a control electrode, of the type which may be triggered toconduction upon the application of a control signal across the controlelectrode and one of the current carrying electrodes thereof. In thefigure, this controllable switching device is illustrated as a siliconcontrolled rectifier 40, the current carrying electrodes of which areconnected in series with braking winding 26, however, it is to bespecifically understood that alternate controllable switching deviceshaving similar electrical characteristics may be substituted therefor.

The silicon controlled rectifier is a semiconductor device having acontrol electrode, generally termed the gate electrode, and two currentcarrying electrodes, generally termed the anode and cathode electrodes,which is designed to normally block current flow in either direction.With the anode and cathode electrodes forward poled, ianode positive andcathode negative, the silicon controlled rectifier may be triggered toconduction upon the application, to the control electrode, of a controlpotential signal of a polarity which is positive in respect to thepotential present upon the cathode electrode and of sufficient magnitudeto produce control electrode-cathode, or gate, current. In theconducting state, the silicon controlled rectifier will conduct currentin one direction and retains the ability to block current flow in theopposite direction. In the conducting state, therefore, the siliconcontrolled rectifier functions as a conventional diode. Upon beingtriggered to conduction, however, the control electrode is no longercapable of affecting the device which will remain in the conductingstate until either the anode-cathode circuit is interrupted or thepolarity of the potential applied across the anode-cathode electrodes isreversed.

As the anode electrode of silicon controlled rectifier 40 is connectedto the positive polarity terminal 17 of braking potential source 14through braking winding 26 land the cathode electrode 42 is connected tothe negative polarity terminal 18, this device is forward poled.

The difference signal may be applied directly across the controlelectrode 41 and cathode electrode 42 of silicon controlled rectifier 40by connecting movable contact 37 of potentiometer 34 to controlelectrode 41 and movable contact 31 of potentiometer 28 to the cathodeelectrode With these connections, the difference signal is of the properpolarity relationship, positive upon movable contact 37 in respect tomovable contact 31, to produce gate current flow through siliconcontrolled rectifier device 40. Therefore, this device is triggered toconduction and establishes an energizing circuit for brake winding 26across the direct current output terminal 17 and 18 of source of directcurrent brake potential 14, upon the adjustment of movable contact 25 ofmotor speed control potentiometer 24 to reduce the speed of motor 12.

It has been found, however, that more precise and reliable operation ofthe novel dynamic braking system of this invention may be realized bythe inclusion of intermediate circuitry responsive to the differencesignal for producing a control signal which is applied across the gateelectrode 41 and cathode electrode 42 of silicon controlled rectifier 40to trigger this device to conduction in response to the differencesignal. This intermediate circuitry includes type NPN transistor 50,unijunction transistor 60 and the associated circuitry.

Type NPN transistor has the usual control electrode, base electrode 51,and two current carrying electrodes, emitter electrode 52 and collectorelectrode 53.

To apply the difference signal across the control electrode and one ofthe current carrying electrodes of transistor 50, movable contact 37 ofpotentiometer 34 is connected to the base electrode 51 through currentlimiting resistor 65 and movable contact 31 of potentiometer 28 isconnected to the emitter electrode 52 through lead 66, potentiometer 57and resistor 56.

The current carrying electrodes, collector electrode 53 and emitterelectrode 52, of transistor 50 are connected in series with a capacitor58 across the positive and negative polarity terminals, respectively, ofa direct current potential source 55 which, since it may be any ofseveral direct current potential sources well known in the art and formsno part of this invention, has been shown in the figure in block form.This circuit may be traced from the positive polarity terminal of directcurrent potential source 55 through the collector-emitter electrode oftransistor 50, resistor 56, potentiometer 57, capacitor 58 and point ofreference or ground potential 5 to the negative polarity terminal ofdirect current potential source 55.

Unijunction transistor 61 and 62 and an emitter electrode 63. Emitterelectrode tive polarity output resistor 68, bases 61 and 62 ofunijunction transistor 60, primary winding 72 and point of reference orground potential 5 to negative polarity terminal 18. Zener diode 71tends to stabilize the potential across base 61 and 62 of unijunctiontransistor 60.

Secondary winding 74 of transformer 70 is connected across controlelectrode 41 and cathode electrode 42 of silicon controlled rectifier 40through leads 76 and 78, respectively.

Upon the adjustment of movable contact 25 of motor speed controlpotentiometer 24 in a direction to reduce the speed of motor 12, theresulting difference signal appearing across movable contacts 37 and 31of respective potentiometers 34 and 28 is applied across thebase-emitter electrodes of transistor 50. As this difference signal isof a positive polarity upon movable contact 37 of potentiometer 34 inrespect to the polarity of the potential upon movable contact 31 ofpotentiometer 2.8, this difference signal is of the proper ship toproduce base-emitter current flow through a type NPN transistor.Therefore, transistor 50 conducts and establishes an energizing circuitfor capacitor 58 from the positive polarity terminal of potential source55, through the collector-emitter electrodes of transistor 50, resistor56, potentiometer 57, capacitor 58 and point of reference or groundpotential 5 to the negative polarity terminal of potential source 55.The charge upon capacitor 58 is applied across the emitter 63-base 62electrodes of unijunction transistor 60.

The unijunction transistor is a semiconductor device having two baseelectrodes and an emitter electrode which is characterized by a highemitter to base resistance with varies with diflerent unijunctiontransistor types, it generally falls Within the range of .50 to .75. Thepeak point voltage is substantially the product of the interbasepotential multiplied by the intrinsic standoff ratio of the unijunctiontransistor being used. Therefore, the peak point voltage may varybetween /2 and of the interbase upon the characteristics of thedeprimary winding 72.

This control signal is applied across the control electrode 41 and thecathode electrode 42 of silicon controlled rectifier 40 through lines 76and 78, respectively. Therefore, silicon controlled rectifier istriggered to conduction to establish an energizing circuit for brakewinding 26 across the direct current output terminals 17 and 18 of brakepotential source 14 in response to the difference signal appearingacross movable contacts 31 and 37 of respective potentiometers 28 and34.

With brake winding 26 energized, the rotor of motor 12 is dynamicallybraked in a manner well known in the art and, therefore, the speed ofthe rotor of motor 12 tends to closely follow the adjustment of movablecontact 25 of motor speed control potentiometer 24.

Upon the discharge of capacitor 58 through the circuit hereinabovedescribed, unijunction transistor 60 sponmovable contacts 37 and 31 ofrespective potentiometers 34 and 28.

At the end of each half cycle of the alternating current supplypotential, the direct current potential appearing across direct currentoutput terminals 17 and 18 of brake potential source 14 goes to zeroand, therefore, extinguishes silicon controlled rectifier 40.

As long as a difference the cycle hereinabove described repeats to applydynamic brake energizing current to brake Winding 26.

As the speed of motor 12 reduces, the magnitude of the direct currentoutput potential of tachometer 33 also reduces until the potentialacross movable contacts 37 and 31 of respective otentiometers 34positive polarity terminal of direct current reference potential source32. Under these conditions, the ditference signal, which is of anegative polarity upon movable contact 37 in this invention isinoperative.

Upon the adjustment of movable contact 25 of motor speed energizebraking winding 26.

Throughout this scope of the appended claims.

What is claimed is as follows:

1. A dynamic braking system for alternating current induction typemotors comprising in combination with a variable voltage motor speedcontrol system including a motor speed control potentiometer having amovable contact and an associated controlled alternating currentinduction type motor, a source of direct current brake potential, abrake winding wound upon the stator of said motor, a source of directcurrent reference potential, a direct current tachometer generatordriven by said motor, a first potentiometer having two end terminals anda movable contact, means for connecting said end terminals of said firstpotentiometer in shunt across said source of direct current referencepotential, a second potentiometer having two end terminals and a movablecontact, means for connecting said end terminals of said secondpotentiometer in shunt across said direct current tachometer generator,means for mechanically interlocking said movable contacts of said motorspeed control potentiometer and said first potentiometer, and meansresponsive to a difference signal appearing across said movable contactsof said first and second potentiometers for establishing an energizingcircuit for said brake winding across said source of direct currentbrake potential.

2. A static braking system for alternating current induction type motorsas described in claim 1 wherein said means responsive to said differencesignal for establishing an energizing circuit of said brake windingacross said source of direct current brake potential comprises atransistor device having two current carrying electrodes and a controlelectrode, means for applying said difference signal across said controlelectrode and one of said current carrying electrodes of said transistordevice, a capacitor, a source of direct current potential, means forconnecting said current carrying electrodes of said transistor deviceand said capacitor in series across said source of direct currentpotential, a transformer having primary and secondary windings, aunijunction transistor device having two base electrodes and an emitterelectrode, means for connecting said emitter electrode to a junctionbetween said current carrying electrodes of said transistor device andsaid capacitor, means for connectdevice and said primary winding of saidtransformer in ing said base electrodes of said unijunction transistorseries across said Source of direct current brake potential, acontrollable switching device having two current carrying electrodes anda control electrode of the type which may be triggered to conductionupon the application of a control signal across said control electrodeand one said current carrying electrodes of said controllable switchingdevice in series with said brake winding across of said current carryingelectrodes, means for connecting said source of direct current brakepotential and means for connecting said secondary winding of saidtransformer across said control electrode and one of said current carrying electrodes of said controllable switching device.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,408,550 I October 29, 1968 Donald E. Graham It is certified that errorappears in the above identified patent and that said Letters Patent arehereby corrected as shown below:

Column 5, line 47, cancel "peak point voltage. With an applied emitterpotential of". Column 7, line 24, "of" should read for Column 8, line 8,beginning with "transistor device cancel all to and including"transistor" in line 10, same column 8 and insert transistor device andsaid capacitor,

means for connecting said base electrodes of said'unijunction transistordevice and said primary winding of said transformer in line. 16,beginning with "said current" cancel all to and including "forconnecting" in line 18, same column 8 and insertof said current carryingelectrodes means for connecting said current carrying electrodes of saidcontrollable switching device in series with said brake winding across(SEAL) Signed and sealed this 10th day of March 1970.

Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents

